In general, the paper conveying mechanisms used in copiers, printers or facsimile machines have two rubber rollers that have a greater friction coefficient. One serves as a pickup roller and the other serves as a feed roller. The paper located on the top layer is firstly separated by the pickup roller, and is transferred by the feed roller to the printing module for printing. The driving power source usually is located between the two rubber rollers. A drive element, such as a gear set or belt, is used to transmit the driving power to the two rollers.
Nowadays, miniaturization and integration is a prevailing trend. Either single function machines such as printers, copiers, FAX machines, or Multi-Function Peripheral (MFP) that integrates the functions of copying, printing, facsimile or scanning have to rearrange and reconfigure the structure to catch up with this trend. Hence to simplify the existing paper pickup or feeding mechanism has become unavoidable.
Using the paper pickup mechanism that has gear sets to transmit driving power to the pickup roller, there's friction between gears. As a result, energy is lost and noise is generated when the gears transmit the driving power. Therefore, it is preferable to have as few gears as possible. However, this is not always attainable in a practical mechanism. In a conventional power transmission design, it is not uncommon to find more than ten gears of different sizes. There are several reasons why. One is the constraint of a gear transmission path in the structural design, and the other is to enable the pickup roller to have automatic compensating capability.
Take U.S. Pat. No. 5,527,026 assigned to LEXMARK Co. of U.S.A. for instance. Its drive roller 13 is pivotally engaged with a gear located on the bottom of a gear train 1. A drive gear 3 located at the front end of the gear train is stationary. The entire gear train 1 and the drive roller 13 are rotatable according to the amount of paper so that the drive roller 13 is always located on the surface of the paper on the top. When all the gears in the gear train 1 rotate and drive the drive roller 13, a torque is generated to enable the driver gear to exert a normal force to the paper on the top. The normal force is alterable depending on the properties of the paper, such as variety of weight, density and stiffness. Thus an automatic compensation pickup force is generated according to different properties of the paper, therefore multi-feeds or feeding failure of paper may be avoided.
In other words, if the drive roller is pivoted on a “swing arm” such as the gear train mentioned above, the pickup roller is constantly in contact with the paper on the top. Then no matter how much paper is in the paper tray, the normal fore alters according to the properties of the paper.
As the driving power source is located between the pickup roller and the feed roller, as indicated in U.S. Pat. No. 5,527,026 to LEXMARK Co., the driving power source must be located on the paper feeding direction 17. Hence the shortest driving power transmission path should have a drive gear 3 of a gear chain 1 to be located between the drive roller 13 and the driving power source. On the contrary, in the cited reference, the driving power must be transmitted to the drive gear 3 located at the remote location. Thus it needs more gears. As a result, power loss and noise generation are more severe.